1. Field of Use
The present invention relates to a power transmission system for a four-wheel drive vehicle and to control means therefor.
The power transmission system, which is connected to the vehicle engine to drive the front and rear vehicle wheels, comprises a shiftable change gear transmission and an inter-axle differential, including a biasing clutch. The control means sense slippage between the front and rear vehicle wheels and effect temporary clutch lockup to cause the front and rear wheels to rotate at the same speed.
2. Description of the Prior Art
U.S. Pat. No. 3,894,446, issued July 15, 1975 to Snoy et al entitled "Power Transmission Having Friction Clutch Bias Differential With Automatic Clutch Release" and assigned to the same assignee as the present invention, discloses a power transmission system having a change gear mechanism and an inter-axle differential mechanism. That patent discloses a hydro-mechanical control means, including a valve, to sense clutch slipping and to effect disengagement of the clutch in the event thereof. More specifically, U.S. Pat. No. 3,894,446 discloses a power transmission for a four-wheel vehicle in which both the front and rear wheels are driven by the front and rear output shafts, respectively, of the inter-axle differential. That differential is equipped with a modulatable slippable friction clutch that provides a predetermined amount of torque differential or torque bias between the two output shafts of the differential for accommodating slipping conditions. The amount of the torque bias is determined by the load capability of the drive train which is located downstream of the differential, and the clutch serves to protect the drive train from excessive loads. Thus, when the predetermined torque value is attained, the clutch will slip. Control means are provided for sensing the slipping of the friction clutch and operate to completely release (i.e., disengage) the friction clutch prior to the attaining of high slip speed values. This control recognizes rotational speed differences between the two (front and rear) drive shafts and, as a result, releases the friction clutch when a predetermined torque value develops, thereby preventing continuous slip of the clutch at high slip speed values and rated torque which would otherwise cause failure of the clutch.
The prior art also includes U.S. Pat. No. 4,552,241, issued Nov. 12, 1985 to Suzuki entitled "Four-Wheel Drive System With Center Differential Lock Control Responsive to RPM Difference" and assigned to Nissan Motor Company, which discloses a four-wheel drive system for a vehicle which has a center differential between the front wheels and rear wheels, and means for restraining or locking the center differential. The four-wheel drive system is combined with a control system for automatically locking the center differential when the difference between an average rotational speed of the right and left front wheels and an average rotational speed of the right and left rear wheels becomes equal to or larger than a predetermined value. The control system in U.S. Pat. No. 4,552,241 is relatively complex and employs five different sensors, namely: a front right wheel RPM sensor; a front left wheel RPM sensor; a rear right wheel RPM sensor; a rear left wheel RPM sensor; and a condition of center differential sensor.
Other U.S. Pat. Nos. illustrating the state of the art are: 3,748,928; 3,792,628; 4,467,886 and 4,761,043.